1. Field of the Invention
The field of this invention relates to methods and apparatus for an envelope tracking system, and in particular to methods and apparatus for improving an efficiency of an envelope tracking system for a power amplifier module, for example within a radio frequency (RF) transmitter module of a wireless communication unit.
2. Background of the Invention
A primary focus and application of the present invention is the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, the low efficiency is also recognised as being problematic for the base stations.
Additionally, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearising techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion.
Voltages at the output of the linear, e.g. Class AB, amplifier are typically set by the requirements of the final RF power amplifier (PA) device. Generally, the minimum voltage of the PA is significantly larger than that required by the output devices of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output devices, as well as any excess voltage across any pull-down device components due to the minimum supply voltage (Vmin) requirement of the PA.
In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-bit quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity. To help overcome such efficiency and linearity issues a number of solutions have been proposed.
One known technique, as illustrated in the block diagram 100 of FIG. 1, relates to controlling the supply voltage 120 provided to the power amplifier 140. The illustrated technique is known as average power tracking (APT). With APT, an average power level 105 of the transmitted signal is determined and applied to an APT-Vpa mapping module 110 that determines a supply voltage (Vpa) 120 to be applied to the PA 140 based on the determined average power level. This signal is then applied to a DC-DC converter 115 and the resultant (output) voltage is applied to the PA 140 as its supply voltage (Vpa) 120. This technique is known to provide high efficiency, but the speed of signal tracking is limited. Hence, DC-DC converters are typically used in average power tracking (APT) designs to accommodate signal tracking. One known problem with this technique is that APT operates with less efficiency at the higher output power levels when the peak to average power ratio (PAPR) back-off is large, which is predominantly the case for more complex modulation schemes.
Another known supply voltage technique 200 is envelope tracking (ET), illustrated in FIG. 2, which relates to modulating the radio frequency (RF) power amplifier (PA) supply voltage (Vpa) 220 to match (e.g. track) the envelope of the radio frequency waveform being transmitted by the RF PA 240. Typically, ET systems control the RF PA supply voltage 220 in order to improve PA efficiency through selecting a lower supply voltage dependent upon an instantaneous envelope of the input signal. ET systems are often also designed to improve linearity by selecting a RF PA supply voltage 220 dependent upon a constant PA amplification gain. A digital (quadrature) input signal 202 is input to an RF transmitter 230, whose output provides an input power level 235 to the RF PA 240. Concurrently, the digital (quadrature) input signal 202 is applied to an envelope detector 204 arranged to determine a real-time envelope of the signal to be transmitted (e.g. radiated). The determined real-time envelope signal output from the envelope detector 204 is input to an envelope mapping function 210, which is arranged to determine a suitable PA supply voltage (Vpa) 220 to be applied to the PA 240 in order to substantially match the instantaneous real-time envelope of the signal to be transmitted. The output from the envelope mapping function 210 is input to a delay control function 212 that aligns, in time, the PA supply voltage (Vpa) 220 to the signal being passed through RF transmitter 230. The output from the delay control function 212 is input to a supply modulator 214 that provides the PA supply voltage (Vpa) 220 to be applied to the PA 240.
With ET, the instantaneous PA supply voltage (Vpa) 220 of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) of the transmitted RF signal. Thus, since the power dissipation in the PA 240 is proportional to the difference between its supply voltage and output voltage, ET may provide an increase in PA efficiency, reduced heat dissipation, improved linearity and increased maximum output power 225, whilst allowing the PA to produce the intended RF output. However, the total system efficiency is affected by supply modulator efficiency that is related to the supply modulator design, supply voltage range, bandwidth and PA loading, which typically results in ET modulator efficiency not being high enough for most applications. The envelope mapping function 210 between ENV and Vpa is critical for optimum performance (efficiency, gain, and adjacent channel power (ACP)). Also critical to system performance is timing alignment between the RF signal and Vpa at the PA.
A yet further known technique 300 is to combine envelope tracking (ET) with digital pre-distortion (DPD), as illustrated in FIG. 3. Here, control/manipulation of the input waveform/signal in the digital domain is performed in order to compensate for PA nonlinearity (AM-to-AM and AM-to-PM) effects, thereby improving PA output linearity based on prior information or measured data of the PA system. Again, a digital (quadrature) input signal 302 is input to an RF transmitter 330 via a digital pre-distortion (DPD) function 326, whose output provides an input power level 335 to the RF PA 340. Concurrently, the digital (quadrature) input signal 302 is applied to an envelope detector 304 arranged to determine a real-time envelope of the signal to be transmitted (e.g. radiated). The determined real-time envelope signal output from the envelope detector 304 is input to an envelope mapping function 310, which is arranged to determine a suitable control voltage (Vdc) 320 to be applied to the PA 340
In this manner, envelope-tracking can be combined with digital pre-distortion (DPD) on the RF signal to improve adjacent channel protection (ACP) robustness. However, since the ET system is often a multichip implementation involving function blocks in digital baseband (BB), analogue BB, RF transceiver, power management and PA, consistent ET system performance cannot easily be guaranteed across all devices by hardware.
The overall transmitter efficiency is, in large part, dependent upon the efficiency of both the PA and the supply modulator path. In particular, the inventors have recognised that the efficiency usually decreases as the input signal bandwidth increases.
A number of other modulator designs are known. For example, a linear regulator/modulator design may be used, whereas although signal tracking is fast it is known to suffer from poor efficiency. As a result of the poor efficiency, it is rarely, if ever, used for ET applications. Another example is a hybrid modulator, which comprises a switching modulator and linear amplifier. In hybrid modulators, most of the envelope energy is delivered by the switching modulator, whilst the wide bandwidth of the envelope signal is supported by linear amplifier. However, the linear amplifier needs to accommodate large envelope bandwidths and also suppress switching noise. These requirements have an adverse impact on the hybrid modulator efficiency.
A paper titled ‘Slew-rate limited envelopes for driving envelope tracking amplifiers’ (by Gabriel Montoro, et al. and published by the Dept of Signal Theory and Communications by the Universitat Politecnica de Catalunya in Barcelona, Spain), describes a technique that sets a maximum value for a slew-rate limiter in an ET path, but has been identified as introducing some delays and out-of-band emissions of the supply modulator under varying load conditions.
A yet further paper titled ‘A DSP structure authorizing reduced-bandwidth DC/DC Converters for Dynamic Supply of RF Power Amplifiers in Wideband Applications’ (by Albert Cesari, et al. and published by the Groupe Integration de Systemes de Gestion de l'Energie in Toulouse, France), describes a technique that tracks a peak of an original envelope signal, but has been identified as also introducing some delays and creating poor out-of-band emissions of the supply modulator due to use of a DC/DC converter input step signal.
Thus, there is a need for a more efficient and cost effective solution to the problem of improving the overall transmitter efficiency, and in particular the supply modulation efficiency.